Meridional variations in orientation discrimination in normal and amblyopic vision

Orientation discrimination for single long lines presented in a frameless environment was measured with a method of constant stimuli in 18 normal subjects and in both eyes of 9 amblyopes. Orientation discrimination was tested at four meridians (horizontal, vertical, left, and right oblique). Although the normal subjects showed considerable individual variability in their just noticeable differences in orientation, each subject showed a consistent oblique effect. In amblyopic subjects, the interocular differences were strongly meridian-dependent and individually variable. Across amblyopic subjects, a two-fold increase in just noticeable differences ( JNDs ) was observed for the principal meridians, while the impairments were not significant for the oblique meridians. These small impairments in orientation discrimination strongly contrast with the high losses in acuity for the same subjects, which suggests that different mechanisms underly acuity and orientation discrimination.